Scientists have made a breakthrough in understanding the movement and structure of ion hydrates

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Czech scientists, in cooperation with Chinese colleagues, were the first to show the structure and determine the mobility of ionic sodium hydrates, clusters made of water molecules and sodium atoms, involved in a number of significant physical, chemical and biological processes. Thanks to a special method, they confirmed that the mobility of these miniature objects, which affects their effectiveness, is related to the internal arrangement. The study was published by Nature magazine these days [1].

Pavel Jelínek from the Institute of Physics of the Czech Academy of Sciences and the Regional Center of Advanced Technologies and Materials of the Palacký University in Olomouc took part in the work. According to him, the author's team managed to fill one of the white spaces in chemistry, biology and physics. Hydration of ions on surfaces is of great importance, for example, in corrosion, electrochemistry, or transport of ions in living organisms. Without ion hydrates, cells could not function, or dissolving salts, such as ionic beverages.

"We have taken another step to understanding how hydrates work and how their structure affects their mobility. This is an important advance, no one else has been able to study these substances, their movement and structure with such precision," said Jelinek, an internationally renowned expert in theoretical and experimental studies of physical and chemical properties of molecular structures on solid surfaces using scanning microscopes.

Chinese colleagues first prepared different types of clusters when different numbers of water molecules bound to the atom of sodium. The results of these atomic games were subsequently examined by scientists using specially modified scanning microscopes. However, a fundamental discovery would not have been possible without a special method, the development of which was greatly influenced by Czech physicists, and published this year by Nature Communications magazine in January [2].

"The difficulty in studying these clusters is their relatively weak internal connections, which easily break the tip of the microscope. Using a new imaging method, we can overcome this obstacle by hanging a single carbon monoxide molecule on the tip apex. Thanks to its presence, we are able to see not only whether the ion surrounds one, two or even three water molecules, but we also monitor the arrangement of individual molecules without disrupting the structure of the studied ion hydrate cluster," Jelinek explained.

When scientists knew the composition and form of the observed clusters in detail, they pulled the electric pulse and measured their mobility. In general, the higher the mobility of clusters, the more effective they are. "We found that the sodium ion hydrated with three molecules of water was the fastest. This means that this type of cluster will play an important role in the ability to manage sodium transport in biology or to influence corrosion," Jelinek added.

The ability to see individual molecules, manipulate them and even show chemical bonds was imaginable a few years ago. This has been facilitated by the development of atomic resolution scanning microscopes that have opened up new possibilities for scientists, among other things, in the field of characterization and modification of nanostructures.

A schematic illustration of the induced movement of the ionic hydrate towards the tip of the microscope by means of an electric pulse. The research has shown that an ionic hydrate consisting of a single sodium atom (purple ball) surrounded by three molecules of water (hydrogen-white ball, oxygen red ball) shows significantly greater mobility than other types of hydrate formed by a different number of water molecules. This finding demonstrates that the mobility of ion hydrates is greatly influenced by their structure.

[1] J. Peng, D. Cao, Z. He, J. Guo, P. Hapala, R. Ma, B. Cheng, J. Chen, W. J. Xie, X.-Z. Li, P. Jelínek, L.-M. Xu, Y. Q. Gao, E.-G. Wang, and Y. Jiang, The effect of hydration number on the interfacial transport of sodium ions, Nature (2018)
[2] J. Peng, J. Guo, P. Hapala, D. Cao, R. Ma, B. Cheng, L. Xu, M. Ondráček, P. Jelínek, E. Wang, Y. Jiang, Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy, Nature Communication 9 (2018) 122(1) - 122(7).